The invention relates to a method of operating a cellular telecommunication system wherein one mobile station may communicate with a number of base stations in a macrodiversity mode, and wherein each of the base station is provided with a target signal-to-interference (SIR) value. The invention also relates to a corresponding cellular telecommunication system.
In Direct Sequence Code Division Multiple Access (DS-CDMA) cellular telecommunication systems, a mobile station may communicate in a macrodiversity mode with a number of base stations. Due to the several established links in this mode it is possible that one of the base stations runs into a high load situation in which the base station is not able anymore to accept any mobile station as a further participant of the telecommunication system. The availability of service for the users of the system is thereby reduced.
It is an object of the invention to provide a method for operating a telecommunication system which ensures a high performance of the system in a macrodiversity mode.
For a method as described above, this object is solved according to the invention in that an actual load is provided for each of the base stations, and that the target SIR value of that base station is increased which carries a high actual load. For a telecommunication system as described above, this object is solved according to the invention by a radio network controller for providing an actual load for each of the base stations and for increasing the target SIR value of that base station which carries a high actual load.
The increase of the target SIR value has the consequence that the respective base station requests an increase of the transmission power level of the connected mobile stations. However, if one of these mobile stations is also connected to another base station in the macrodiversity mode, then this mobile station will probably not increase its transmission power level due to the connection with the other base station. This has the result that the actual SIR value of the link between this mobile station and the highly loaded base station will remain below the target SIR value.
In an advantageous embodiment of the invention, the actual SIR value is provided for any link between a mobile station and a base station, and that link is released which has a low actual SIR value.
As described, the actual SIR value of the link between that mobile station which is connected to several base stations in the macrodiversity mode, and that base station which carries a high load, is below the target SIF value. As a consequence, this link is released. The load on the respective base station is thereby reduced. However, the communication as such is not disconnected as the released link is only one connection of a number of connections of the macrodiversity mode of the respective mobile station.
The invention, therefore, has the advantage that, on one hand, the load on an overloaded base station is reduced, and that, on the other hand, only links of the macrodiversity mode are released so that no communication as such is disconnected.
It is advantageous that the actual load of each of the base stations is compared with a target load, and that the actual load is determined to be high if it exceeds the target load. As well, it is advantageous that the acutal SIR value is compared with the target SIR value, and that the actual SIR value is determined to be low if it is below the target SIR value.